Research and development of organic electroluminescence devices has been actively conducted in recent years because highly luminescent emission is obtained from these devices with low-voltage driving. In general, organic electroluminescence devices are constituted of an organic layer including a light-emitting layer, and a pair of electrodes between which the organic layer is sandwiched, and electrons injected from the cathode are recombined with holes injected from the anode in the light-emitting layer, to produce excitons, whose energy is utilized to luminescence.
Improvement in the efficiency of devices has recently made by using a phosphorescent materials. For instance, an organic electroluminescence device using iridium complexes or platinum complexes or the like as a phosphorescent material and therefore having improved luminous efficiency and heat resistance have been studied.
In addition, dope type devices using a light emitting layer obtained by doping a light emitting material in a host material have been adapted widely.
US 2008-297033 describes an invention using a condensed-ring type phosphorescent material with a view to obtaining a device capable of emitting blue light, having excellent durability, having a sharp emission spectrum, and low power consumption. However, such devices using the condensed-ring type phosphorescent material has a low external quantum efficiency.
On the other hand, development of host materials has been active. For instant, JP-A-2009-99783 discloses an invention using a condensed-ring type polycyclic aromatic material as a host material in order to fabricate a device with high efficiency and long lifespan. JP-A-2009-99783 also describes an invention using an electron transporting material having a carbazole structure. According to it, however, introduction of materials which can produce an unstable oxidized species having carbazolyl groups is not preferred because it leads to shortening of the lifespan of the device.
In addition, the conventional devices are required further improvement because they do not have a sufficient luminous efficiency and moreover, the luminous efficiency may depend on the thickness of their electron transporting layer.
As described in JP-A-2009-99783, it is known that the use of materials which can produce an unstable oxidized species having carbazolyl groups is unfavorable to durability of devices. In light of such common knowledge, embodiments of the present invention couldn't be expected to have effects on durability improvements.
In addition, conventional devices do not have a sufficient luminous efficiency and moreover, the luminous efficiency may depend on the thickness of the electron transporting layer so that they are required for further improvement.
The present inventors have however found that when a host material containing carbazolyl group according to the invention is used in combination with a specified condensed-ring type phosphorescent material, a device excellent in luminous efficiency can be obtained and moreover, dependence of the luminous efficiency on the film thickness of the electron transporting layer can be reduced.